This invention generally relates to semiconductive photo sensors, and is specifically concerned with a high speed, high power density optical sensor formed from a plurality of optical fibers disposed between the substrates of adjacent segments of a vertical FET Field Effect Transistor, each of which serves an elemental transistor..
Optical photo sensors are well known in the prior art. One prior art design comprises a planar FET having a bottom, heavily doped region electrically connected to a source and a top, lightly doped region that includes a depleted region which is covered by a drain electrode. The drain electrode is typically formed from a layer of gold which is thin enough to transmit light In operation, a biasing voltage is applied across the bottom, heavily doped region of the FET and the drain electrode while a signal-carrying beam of light is focused on the depletion region. Photons from the light beam penetrate the drain electrode and create electron-hole pairs in the depleted region which has the effect of modulating the conductivity of the FET commensurate with the amplitude of the light beam. The resulting pulsating flow of current through the FET is converted into alternating current by means of a bypass capacitor and amplified.
The applicant has observed that the performance of such prior art photo sensors is limited by three factors. The first of these factors is the inefficiency of the optical coupling between the signal carrying light beam and the depleted region of the FET. Despite the thinness of the drain electrode, much of the signal-carrying light is reflected from this electrode before it ever has an opportunity to penetrate the depleted region. In an attempt to solve this problem, some prior art designs have provided a hole about 2 mils. in diameter which provides a direct light path into the depleted region. However, experience has shown that is very difficult to accurately focus a light beam into such a small hole. Hence, much of the signal-carrying light beam is absorbed into the undepleted regions of the semiconductor substrate. A second factor that substantially limits the performance of such prior art optical sensors is the capacitative leakage that occurs between the source and the drain at light frequencies at 10.sup.6 hertz and beyond. Such capacitative leakage appears to be an intrinsic limitation of the planar topography of such prior art photo sensors where, at high frequencies the broad and flat and closely spaced source and drain electrodes disposed across the semiconductor material act as a capacitor. The third factor limiting the performance of such prior art photo sensors is the limited power density available from such designs. Like the unwanted capacitance that limits high speed performance, the power density limitation again stems from the planar topography of the FETs used in such photo sensors. This limitation as to power density is a significant drawback where micro-miniaturization is desirable, such as in satellite communication circuitry.
Clearly, what is needed is a higher efficiency optical sensor which provides better coupling between the signal-carrying light beam and the photo sensor elements. Ideally, the processing speed of the photo sensor should not be limited by unwanted capacitance at light frequencies of 10.sup.6 hertz and over. Finally, it would be desirable if the optical sensor were capable of substantially higher power densities